Normally, I have the planes on a wall charger which is on a timer for 2 hours per day so the plane is ready anytime. Last weekend, one plane was off the plug for 2 days so at the field I checked and was surprised to see voltage at 4.8 so I topped it up and it took 200 mAh. I was surprised I would loss that much in 2 days.
So I did a test the past few days; charged 2 planes up (both have the same battery ; 4.8V, 1100 mAh). Both were at 5.1V when done. I unplugged and watched the voltage. Both dropped the same but were at 4.7 after 2.5 days. My rule of thumb is 1% loss per day for NiCd so this surprises me. I do not think I have a bad cell as both batteries are doing the same thing and at the field, I can fly 3-4 flights and still be above 4.8V.

Is this normal? Surprising? The only thing I can think of is to cycle them as I am likely only using <200mAh each time out and then top them back up...

Thoughts appreciated

NiCad will loose about 1 to 1.5% per day, NiMh about twice that with typical batteries. Now some of the newer battery technologies claim to have improved on that; for instance Sanyo Eneloops (which are NiMh) are supposedly almost as good as LiPo and Pb for long term retention of charge. Tests seem to confirm that fact.

Chemie,

I'm not sure if NiCads can develop a memory. If you are charging them every day without discharging them, you may be inducing a memory in them. Try running them through a couple of discharge/charge cycles. This may rejuvinate them.

There isn't a direct relationship between capacity and voltage but it does seem as though these packs are discharging.
Are they completely removed from all connections during the period between measurements?
Are they contaminated with something, like fuel, that could be causing a gentle discharge?
Are they kept in a warm place?
A good cycling wouldn't hurt them.

When I make my own packs I use the eneloops and they hold there charge. If you can I would cycle your batteries a few times to test just to make sure there is no problem. I have found the NIM battery needs to be cycled about four times to come up to it's full capacity when new. They aren't supposed to retain a memory but a cycle or two doesn't hurt.

How much time should a person be getting out of an 1100 mah 4.8 4 cell pack?  I get about 4(maybe a little more) out of my 600 and was going to buy some 1100 nicds, but dont want to if you cant get much more out of them.</p>

They are still hooked to the harness but the switch is off; no obvious shorts

Not sure how much time 4 is but with a 4.8 volt 2000mah with 5 or 6 digital servos I get about 6...........................12 minute flights depending on what type of flying I'm doing. That's in a 68 inch 13 pound Extra. In a trainer with the same pack and 4 standard servos I can fly most the day. I do have a volt meter and check after every flight though. If your getting 4 then you may get 8??? I also have a field charger so I never let my packs get too low. If your happy with a 600mah then keep using it but when you need a new pack try something a bit bigger and see for yourself. Another thing to think about is what type of charger do you have and how much added time will it take to charge a bigger pack. My favorite charger with handle 6 batteries at a time but only charges to 1500mah. I now tend to use that as my trickle charger and use my other chargers for the main charge.

NiCads and NiMH will lose 5% to 10% or more (NiMH being a little worse than NiCad) in the first few hours off the charger and then level out to 1% to 2% per day.

My transmitters will drop from 11.6v to 11.2v in the time it takes to unplug them and drive 20 minutes to the runway.

I have a raptor 30 heli that i have a sanyo 4.8v 1100MaH battery in and i couldnt get the charge to last more than about a day it would be almost completly dead. they do get a memory, i have charged it up and just left the switch on while im at work to completly dishcarge it as i do not have a good charger yet. then when i got home i would recharge it and then monitor how much it drops 12 hrs after disconnecting it. well after about 3 complete drains and recharges it seems to be working optimally again. I would drain them completly dead and recharge a couple times they should get better but some discharge is normal.

I flew three times yesturday and it only dropped to 4.9...so why would it be capacity or memory?

First, cut the daily charge down to 1 hour per day. The longer charge time is only making heat. The other thing is you did not say what kind of chemistry your packs are, though I assume they are NiCads.

Finally, measurement of how much charge to top off is not that important. Measurement of remaining capacity is what you want to know. Time to discharge to 4.400 Volts will tell you if you have a weak cell if discharged at 300 to 500 mAh. If you plot Voltage vs time it should be a fairly straight line after the initial surface charge is dumped. When it goes over the knee suddenly, one or more cells have no more current capacity. A very well match pack will go as low as 4.3 Volts before you hit the knee of the discharge curve. If it happens at higher voltages, then the pack is not a healthy pack.

Presently, I am working on a project using low self-discharge NiMH cells. During discharge, the cells drop from 1.48V immediately after charge to 1.3V in 5 minutes; but they then take a further 225 minutes to reach 1.05V which is complete exhaustion (for the purposes of the standard).
Immediately at end of charge (EOC) the cells are between 1.47 and 1.49V, dependant upon the cell and temperature,but if left alone will fall to 1.38V within an hour.
This voltage drop just after charge isn't anything to bother about unless it's grossly aberrant.

I cycled the batteries. One was at 700 mah capacity but after a few cycles was back to 950 mah. The other started at 400 mah but after 4 cycles was also back in the 950 mah range.

I checked the voltage during discharge at 300 mah rate (attached) and it seems fine.

So to summarize, it will charge and discharge normally.

However, when I let it sit for 3 days, open circuit, it loses 30% capaity. It took 266 mah of charge after being peaked out 3 days before. This sounds strange to me as NiCad should lose 1%/day and NiMH 2-4%/day.

Anyway way to reverse?

As nickle based batteries age, they can grow dendrites (crystal formations) that can penetrate the thin separation between the plates of the cells which causes leakage. Once this happens, no permanent fix is reliable. You can burn the partial short out with a high current pulse but the fix is only temporary as the dendrites grow right back at the same puncture in the separation material. Many years back a highly touted fix (never was a reliable one) was to charge a very large capacitor to 12 volts or more and then put it across the battery terminals. The momentary high current would burn out the dendrite and every thing appeared to work well but; unfortunately for a very short time.

Shouldnt we move to LiFe (or A123) batteries? I recently bought a 3200mAh from Hobbico and it just doesnt seem to discharge

Ameyam

There is no set amount of time.

The time you get depends upon how much current you are drawing from the battery pack.

Unfortunately many RC'rs grossly underestimate what their electronics will draw.

A single standard servo is capable of drawing up to 2A ( 2000mA ) from your 1100mAh pack at full stall.

That means if a single servo bottoms out or has restricted movement, it can cause your available voltage to drop to the point that the other servos may stop functioning temporarily and/or your receiver may stop working!

Fortunately such high individual servo draw is rare on a properly configured plane, but you should really allow for as much as 200-400mA to be drawn by a single standard servo under in-flight loads.

So assuming five servos... 200mAh x 5 = 1000mA... in other words a 1100mAh pack MAY be just able to keep up with this for short durations ( what we typically see... ) on a non-agressively flown plane.

On any plane larger than .50 I don't use anything less than 1600mAh sized packs, and typically I'll go with 2000mAh up to .90's and 3600mAh packs beyond that.

I've measured current draw on a typical .46 size plane to be about 200-280mA with NO stick movement or servo center hunting...

That means the plane could possible sit completely still, but powered up, for three hours using a 1100mAh pack... Once you are in the air, things change.

On my gassers, I'll field charge the packs after every 4th flight or so if I'm using 2000mAh packs just to be safe.

I have slow chargers and a peak detection field charger for my existing Nickel XX packs, and I have a loaded volt meter. I can test and charge as necessary without buying anything. My chargers and cycler do not support Lithium; and I am not willing to make a new investment at this time.

Carrell

It's just wearing out. Try replacing it.